Outcropping strike-slip faults in Cretaceous peritidal limestones (Altamura Fm.) display different type of calcareous breccia present either within isolated pockets or continuous volumes surrounding the main slip surfaces. Furthermore, original neptunian dikes and sheared neptunian dikes infilled with bauxite-rich matrix and limestone fragments are along walls and pavements of inactive quarries of the Altamura area (Murge area, Italy). In this work, we focus on the distribution, geometry, texture and composition of representative sheared neptunian dikes and tectonic limestone breccias to investigate the role exerted by Cretaceour neptunian dikes on nucleation and growth of strike-slip faults within tight layered limestones. In fact, during post-Cretaceous tectonics some of the 10’s of cm to m-thick neptunian dikes have been sheared, either right- or left-laterally, deforming its original fabric. At the same time, cataclastic deformation of intact limestones took place, at very shallow depths, within the evolving fault zones forming tectonic breccias. The sheared neptunain dikes are sub-vertical open fissures infilled matrix-supported monomictic intraformational clasts embedded within a reddish hematite-bauxite clayish matrix mixed to calcite microsparry cement . Clasts are from cm- to dm-sized, sub-angular to angular and, overall, quite unsorted. In contrast, tectonic breccias consist of either isolated pockets (along m-offset faults) or continuous, at the mesoscale, volumes (10’s of m-offset faults) of cm- to 10’s of cm-sized, angular to sub-rounded clasts embedded within a calcite marix and often reddish bauxite matrix. Generally, the overall clast size decreases in the vicinity of the main slip surfaces. Meso- and microscopic evidence clearly indicate that clasts are due to comminution processes. The results of mineralogic analysis conducted on selected samples of sheared neptunian dikes show that the red matrix and cements are mainly made up of quartz, calcite, kaolinite, hematite and goethite. Differently, the results of similar analyses conducted on samples of tectonic limestone breccias are consistent with calcite and iron oxides. The different mineralogic and textural composition of the two aforementioned elements well portray the variety of breccia distribution along the major, 10’s of m-offset fault zones present in the area. In fact the two different rocks show dissimilar grain matrix ratio and clast shape. The conclusions of this study can be discussed in terms of the control exerted by pre-existing deformation on karst evolution. The study fault zone, actually, show evidences of multiple stages of karst development. Many features such as fracturing, brecciation and cementation suggest that subsurface fluid flow was strongly localized within these evolving fault zones. Faults with presence of remnants of former neptunian dikes can be inferred as a local barriers to fluid flow due to the presence of terrigenous materials chacterised by clay minerals. In contrast, faults in which the fault core is entirely made up of tectonic breccia the abundance of coarse and angular particles ensures efficient transmissibility.

The role of neptunian dikes on strain localization within layered Apulian carbonates, Italy

RUSTICHELLI, ANDREA;TONDI, Emanuele
2014-01-01

Abstract

Outcropping strike-slip faults in Cretaceous peritidal limestones (Altamura Fm.) display different type of calcareous breccia present either within isolated pockets or continuous volumes surrounding the main slip surfaces. Furthermore, original neptunian dikes and sheared neptunian dikes infilled with bauxite-rich matrix and limestone fragments are along walls and pavements of inactive quarries of the Altamura area (Murge area, Italy). In this work, we focus on the distribution, geometry, texture and composition of representative sheared neptunian dikes and tectonic limestone breccias to investigate the role exerted by Cretaceour neptunian dikes on nucleation and growth of strike-slip faults within tight layered limestones. In fact, during post-Cretaceous tectonics some of the 10’s of cm to m-thick neptunian dikes have been sheared, either right- or left-laterally, deforming its original fabric. At the same time, cataclastic deformation of intact limestones took place, at very shallow depths, within the evolving fault zones forming tectonic breccias. The sheared neptunain dikes are sub-vertical open fissures infilled matrix-supported monomictic intraformational clasts embedded within a reddish hematite-bauxite clayish matrix mixed to calcite microsparry cement . Clasts are from cm- to dm-sized, sub-angular to angular and, overall, quite unsorted. In contrast, tectonic breccias consist of either isolated pockets (along m-offset faults) or continuous, at the mesoscale, volumes (10’s of m-offset faults) of cm- to 10’s of cm-sized, angular to sub-rounded clasts embedded within a calcite marix and often reddish bauxite matrix. Generally, the overall clast size decreases in the vicinity of the main slip surfaces. Meso- and microscopic evidence clearly indicate that clasts are due to comminution processes. The results of mineralogic analysis conducted on selected samples of sheared neptunian dikes show that the red matrix and cements are mainly made up of quartz, calcite, kaolinite, hematite and goethite. Differently, the results of similar analyses conducted on samples of tectonic limestone breccias are consistent with calcite and iron oxides. The different mineralogic and textural composition of the two aforementioned elements well portray the variety of breccia distribution along the major, 10’s of m-offset fault zones present in the area. In fact the two different rocks show dissimilar grain matrix ratio and clast shape. The conclusions of this study can be discussed in terms of the control exerted by pre-existing deformation on karst evolution. The study fault zone, actually, show evidences of multiple stages of karst development. Many features such as fracturing, brecciation and cementation suggest that subsurface fluid flow was strongly localized within these evolving fault zones. Faults with presence of remnants of former neptunian dikes can be inferred as a local barriers to fluid flow due to the presence of terrigenous materials chacterised by clay minerals. In contrast, faults in which the fault core is entirely made up of tectonic breccia the abundance of coarse and angular particles ensures efficient transmissibility.
2014
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11581/369010
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